LED UV exposure box part 1, the box

Note: Don’t look directly into the UV LEDs. It harms your eyes.

Introduction

I have been using the toner transfer method for about 9 years with great results. Occasionally I would need a board with finer traces and I would use UV method. My exposure setup allowed only a single sided board to be made and it was composed of a 30cm UV tube, some holders, a table, board and film. Exposure time was rather long as the tube was far away from the board.

A few months ago I was looking for some LEDs at one of my suppliers and found that they also had UV LEDs. My mind went directly to upgrading my not so great setup, so I began looking on the internet for info. It turns out some other people have done it, and this instructable confirmed it was really possible. A LED UV box has a clear advantage over a tube one: it can be made in any size, depending on the needs.

I bought 25 LEDs and began to experiment. This way I was able to find a compromise between LED density (spacing), distance between LEDs and board and cost. I wanted something compact, I don’t make large PCBs and those 30cm tubes were not right for this job.

The LEDs

The LEDs are OSSV53E1A from OptoSupply and they have an 140° angle. This means that the first plane with uniform density can be obtained at a shorter distance from the LED, for a given spacing between LEDs which means smaller height for the board. Their peak wavelength is 405nm, which is higher than recommended by the boards’ manufacturer. Results showed they emit sufficient UV light in the responsive spectrum of the photo resist to allow for a perfect exposure.

I wanted to use a common voltage so 3 LEDs in series with a 91Ω resistor are connected to 12V to ensure a 20mA current.

The final setup will contain a LM317 for stabilizing as I don’t plan on using my variable regulated supply all the time. There is another reason for this, due to the non linear nature of the LEDs, varying 12V with +/-5% creates much larger current variations which influences the required time. Putting a regulator on the box simply allows more flexibility in choosing the power supply.

On the bottom board I have added red LEDs. These LEDs help me align the top and bottom films. I have used low brightness, cheap, matte LEDs as I have to look into them when aligning the films. Connection is similar, 3 LEDs in series with a 220Ω resistor, except for the last row which contains 2 LEDs and a 330Ω resistor. These are not really necessary, aligning the films can be done in a dim ambient light.

The details

For safety reasons and extra functionality I added a switch to turn off the UV light when the box opens, turn on the red light and stop the timer. This makes the box more high tech and gives it a professional behavior.

I used nuts glued on the interior walls of the box to hold the bottom 4 mm thick plexiglas panel. The choice was simply of what was easily available: nuts. Don’t use normal glass as it blocks ultraviolet light. There is no top panel mounted, because it is part of the aligning procedure: I put the first film on the bottom, then the board and the second film is taped to a smaller piece of plexiglas. Aligning means aligning the smaller piece of the plexiglas so that the two layers overlap perfectly. Having the film attached to it makes it easier. Extra markings outside the board help this (ghost vias). Part two should contain a more detailed tutorial on making double sided boards.

The final assembly allows an area of about 10x15cm of board size, with the interior dimensions of the box being 18cm long, 11cm wide and 5.5cm deep for each of the two halves. Exterior dimensions will depend on the thickness of the material used. Each board contains 7×12 UV LEDs, with 3 and respectively 4 holes between them( the actual spacing between the center of the LEDs is the same). The distance between the top of the LEDs and the board is 3.5cm, including the 4mm plexiglas panel. Check out the detail below to see the LED arrangement on a normal 0.1″ or 1.27 spaced board:

Exposure results

I tested with a small board from 2 minutes up to 8 minutes. Starting with the minimum of 2 minutes proved not to be the best idea, but my long exposure time old setup made me choose this value.

What you can see in the picture above, from 2 to 7 minutes exposure if perfect. At 7 minutes some over exposure starts to show up, only in large copper areas, due to the fact that the film was printed with a laser printer. This result is very different from what I have obtained when using a fluorescent tube: below 12 minutes it was clearly underexposed and above 14 it was clearly overexposed, leaving only a small interval where it was right.

So far I have made 4 boards with 2 minute exposure and they came out perfect. When free time will be available I will investigate what the results are for less than 2 minute exposure. As any time between 2 and 7 minutes seems to work, there is no immediate need for a timer. Still, part two should contain a basic circuit for this.

Check out the gallery below for extra details:

The finished panels:

The box:

Panels installed:

Bottom plexiglas panel installed and a nut:

The back:

The safety switch:

RED on:

UV on(switch is not used):

Building your own

In order to build your own you first must decide what the required size is for you. With my setup, boards up to 10 x 15 cm may be exposed. For this, you will need the following materials:

2 prototype boards, 13 x 25 was the smallest size I found that fitted;

1 switch for the lid capable of 1.5A minimum;

2 hinges;

1 box;

misc: screws, nuts, glue etc.

The box may be made of your choice of material. I’ve built it out of two identical halves, interior dimensions 18 x 11 x 5.5cm each. The material is 8mm thick OSB, for which you will require the following parts:

Great project. It’s great people like you sharing your experience & talent that make everyone else who wish to learn better individuals. I was searching for something I could build to kill germs in well water coming into my home. (don’t have any problem, just being careful). They are commercially available but very expensive. Found some info on UV exposure you might need to be aware of. http://www.ehs.washington.edu/rsononion/uvlight.shtm Thanks

Just thought I’d add my two-penneth on this, I built a similar design about a month ago, however I used standard clear case 5mm UV LEDs and drilled a PCB allowing for a much higher density with around 300 LEDs mounted on a 160×160 with SMD resistors mounted on the rear of the PCB. Not expecting fantastic exposure times I produced a test strip with exposure times of 1min, 2min, 3min etc and developing wiped it clean. Subsequently tested again with much shorter times, 10sec, 15sec, 20sec etc.

With an exposure time of only 15 seconds I am getting excellent results, any longer and its overexposed which was quite shocking. Thought it was worth mentioning that with a higher density of LEDs the exposure time is dramatically reduced to near that of a professional-level UV exposure unit, however I do think some better guarding will be necessary as that is probably a highly dangerous amount of UV light being produced.

Thank you for your blog post with details.
I have checked the cheap chinese LEDs from eBay, flat tops, 400nm, 3.2Vf, 20mA, and now my exposure time is only 4 minutes, instead of 15-18 with fluorescent bulb.

But what is more important, that the copper is now etched with copper sulfate evenly, meaning no more overetching, because of different etching times of different corners, and possibility to make traces thinner and nearer. With fluorescent bulb, the best result was 12mil, 10mil was achievable but hard way. 8mil was rarely achievable. Should test what can I achieve now.

With fluorescent bulb I had to print the templates 3 times on the same sheet of film. Luckily I have a photo printer which matches the printout position 4 times out of 5. Now I can print once and the density is enough. There was no overexposure from 3.5 to 6.5 minutes with 30 second steps tested.

Hello Alessandro. I cannot help you, as I did not build that PCB. It is my design, but i don’t know who made it, just hope that it worked well for the builder.
I found a video with someone trying the same thing: http://www.youtube.com/watch?v=WTuF28TqwT0

I think that the best way is to try and experiment a bit, just like I did with exposing the board. Good luck!

I have bought this solder mask and tested with fluorescent bulb.
I had to cure it for 25 minutes from the height of 2cm.

Since I have now made the LED box, and photoresist curing time is now 3 times faster, I guess, the solder mask curing time should be also faster and results more stable. I will test it in the nearest future and write back the results.

The solder mask itself seems to require more curing after development, since it can go off sometimes when soldering large ground pads even with thermals.

On eBay you can find the same mask cheaper when buying larger can, instead of syringe. There is also white solder mask available, which I want to try to use instead of silkscreen.

I have just finished testing the solder mask with LED box.
13-15 minutes are OK. 12- minutes is a clear underexposure and mask go off when developing. Starting from 16 minutes there are bits of overexposure start to appear within thin traces. Starting from 21 minutes is a clear overexposure.

Tests have been done with the same setup. 3.5cm between the board and LEDs tops, including 3mm clear photoframe glass.

This is very good news. I think i might get some solder mask myself. How much time does it take you to make a pcb from start to finish? (assuming you have the design done, of course).
Can you add some links to some pictures with your results?

Made the box (and added one extra row of LEDs just because I’m a mean prick:P) and with 4.5 mins of exposure I can get 0.1 mm continuous traces, so I guess I can say this works just right, so allow me to review this:

PROS:

-“safe”-ish wavelengths for the UV light (which is actually kind of violet at 400-405 nm but is smack in the middle of the photoresist’s maximum sensitivity range. But just play it safe and don’t look at the LEDS while they’re on, or let the light fall on your skin;
-the mentioned LEDs are cheap (got mine from TME – http://www.tme.eu, a Polish supplier) for about 30 Euros for 200 pcs;
-simple construction (just LEDs, resistors and perf board, after that just add power and box and plexiglass);
-If you need 2 amps of 12 volt power, an old computer power supply has all the necessary wires. Just don’t leave it attached to the computer when you use the box;
-tubes are fragile, LEDS aren’t.

CONS:

Hand soldering 200-ish LEDs and the necessary resistors will drive you BANANAS!!!

CONCLUSION:

Cheap, reliable DIY solution, results prove it. Great work and thanks loads for sharing this with us!!!

It’s great that you made a box too. Can you send some pictures so I can add it to the website, or post a link to it?

I’ve got my LEDs from TME too, the price was about the same. You are right about the soldering, it’s very boring, not to mention the red LEDs I’ve used. I actually recommend using a red light bulb in your desk lamp for aligning, it’s much simpler.
On the safe part, mine has no problem, it won’t led the UV LEDs turn on with the box open and the box is not transparent. For the supply part I used local regulation because I had more than one power supply that could be used, so i decided that it was worth to have the box work the same way for various input voltages.

I don’t have anything stronger than 12V at 600mA currently, except for the PC supply, and a 15+V supply at 2A to run through dual LM317s would have cost me quite a lot…

I’ll take pictures when it looks a bit better (right now it runs underneath an IKEA box). I’m gonna experiment a bit with some glass cause I’ve read that it lets long UV through (360nm and beyond). Some thin glass should be rigid enough to keep the film flat against the PCB as well as let most of the necessary UVs through… just keep your fingers crossed, I’ll try it tonight (if I can get some glass cut in time). Of course, this being of awesome importance, results will be posted. 😀

If you will use the same supply with the same load all every time then it is likely that you will be ok. My worries are that if you switch between two 12V supplies, and one gets you 12V-5% and the other 12V+5% you will need to retest for exposure time.
But then again, my results show that there is a rather large interval which gives you a good exposure.

1. Exposure time is now SHORTER. Yes, glass, worked better. 4 minutes for perfect exposure it seems. You may stack more glass to make the film flatter.
2. Making a box is pricey. Unless you’re really enthusiastic about it, recycle something:)
3. Nothing matters more than film-quality it seems. Make it high dpi and high-quality film so it doesn’t curl. One small curl and it’s all over.
4. Watch the developer solution concentration. If you’re not sure, make it weaker, not stronger, and let it cool down before you use it:)

If was possible, use a laminate of crystal, it conducts UV much better. This is the reason why UV lamps are made of crystal, not glass.

Using crystal you will not need to use so much UV LEDs and the exposure time will still the same, or even shorter.

You are always able to find crystal at shops that work with mirrors and special glasses. It is a little bit more expensive than glass, but for a small piece as necessary for this box, probably you will be able to have it for free.

Thanks for the tip on crystal. I knew it was UV transparent but never thought about using it. I will look for suitable glass if plexy will show any signs of yellowing or if boards will appear underexposed.

12 to 14 Minutes with tubes? That sounds odd – my (not-selfmade) exposure box needs 3 to 4 minutes to give good results. Though I found that it well depends on the material used and how old this material is.
For example cheap noname products gave moderate to bad results while some little more expensive material gave me quite good results.

There’s also an expiration date on the material, which is very important – the older the material is, the longer it needs to be exposed to uv light, while “very old” (2 years after expiration date or so) did render the pcb board unusable.

So when “investigating” use equivalent boards (manufacturer and expiration date), otherwise you won’t get comparable results 😉

Sören Sure depends o the tube type and distance. I used single tube at high distance and maybe that is why the time was long.

As for the boards, I can purchase boards from two manufacturers, like I mentioned one is Bungard and I dont know the other’s name, but results are very similar, i never changed the exposure time.
Never had a board stay for as long as two years before being used.

Nice! I used the same LED’s for exposure!
Something about wavelength and dager to the eyes:
(did some research before trying)
The led’s are called UV, but they’re actually just violet, not ultra!
Their wavelength is 405..408nm. (check this with your datasheet if you wish)
UVA starts at 400nm (and below). So no UV. So no UV danger to your eyes.
(UVB is the more dangerouse wavelength, but that’s far away from this wavelength, 300nm as i recall).
The optimum wavelengt of the PCB resist is 410nm, so this is an almost perfect match to the ‘uv’ LED.
That’s why it works so great!
Nice timer you built!

It’s a great thing somebody else has built one with same leds. actually, I’ve got two colleagues who replicated the design minus the timer.
The leds are emitting UV light too, they do have a peak at 405nm. The boards I use come from bungard.de, and have a max sensitivity at 350-400. They work perfectly.
I wanted to add protection after the first experiments i made with these leds, just 1 second of looking at them when on seemed very uncomfortable.

I’m sure it really isn’t required, but have you thought about sanding the tops of the leds to get a more diffused spread? You could sand with something fine like 400 grit and not lose the bevel, so you’d still get the 140 degree spread, albeit a little more diffused and even.

Hi, I was interested to see your UV light box build. You say that you use plexiglass instead of glass, but I thought that glass, although it doesn’t transmit far UV, it does transmit quite a bit of the near stuff.

So it look slike even though normal glass is not such a great transmitter of UV as silica or quartz glass, it may still do better Plexiglass. If that’s the case, you might get a shorter exposure time by switching to glass.

Regarding glass, I have tested with ordinary glass and the time is longer(5 minutes was not enough compared to 2 that I used).
The way I read that graph, you have good transmittance for plexiglass around 400nm.
Still, I need under two minutes to expose the board, which is reasonably short for amateur work.

regarding the transmittance of the glass/plexi for uv, loz is right. however, it is always depending on the thickness of the glass.
e.g. if you compare 6mm of standard window glass to 2 or 3mm of plexi it is obvious that the plexi will be better.

adrian, you are correct, it is dependent on thickness. Also, if that particular thickness of plexi does not extend the time for too long, there is no problem. You have to think of this from an amateur point of view. How much time will you actually save for a single/few board(s) if you exposure time is shorter than my 2 minutes? probably not that much.

While I agree that Plexiglass is reasonably transparent at the wavelengths you are using, your comment earlier that glass is _less_ transparent to UV than plexiglass is incorrect.

The ideal wavelength for UV photoresists seems to be about 365nm. At that wavelength glass is a significantly better transmitter than plexi. 400nm works pretty well and is a long enough wavelength that it doesn’t matter whether you use glass or plexi.

I personally use glass because I can use a thinner panel and get the same rigidity that would require a much thicker plexi panel to achieve.